Retirement Benefits (Schedule of Net Periodic Benefit Cost Not yet Recognized) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Pension and SERP [Member]
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net (Gains) Losses, before Tax	$ 1,014	$ 914
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net Prior Service Cost (Credit), before Tax	17	23
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net (Gains) Losses, before Tax	81	72
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax	2	2
Total amounts not yet recognized as component of net periodic benefit cost	1,114	1,011
Pension and SERP [Member] | Consumers Energy Company [Member]
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net (Gains) Losses, before Tax	1,014	914
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net Prior Service Cost (Credit), before Tax	17	23
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net (Gains) Losses, before Tax	27	22
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax		1
Total amounts not yet recognized as component of net periodic benefit cost	1,058	960
Other Postretirement Benefit Plans, Defined Benefit [Member]
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net (Gains) Losses, before Tax	766	579
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net Prior Service Cost (Credit), before Tax	(132)	(152)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net (Gains) Losses, before Tax	(5)	(9)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax	(3)	(4)
Total amounts not yet recognized as component of net periodic benefit cost	626	414
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net (Gains) Losses, before Tax	766	579
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net Prior Service Cost (Credit), before Tax	(132)	(152)
Total amounts not yet recognized as component of net periodic benefit cost	$ 634	$ 427